1. Field of the Invention
The present invention relates to a laser annealing apparatus and, particularly, to a laser annealing apparatus which is adapted to supply a laser beam of a desired energy value uniformly by automatically correcting the laser energy value resulting from damages in the optical system that occur during application of the laser beam to amorphous silicon layer.
2. Discussion of Related Art
The most widely used thin film transistors for liquid crystal displays have been amorphous silicon thin film transistors (TFT) which are fabricated at a low temperature. In recent years, low temperature polysilicon thin film transistors are considered highly useful in fabricating TFTs because of their high mobility of electrons or holes relative to amorphous silicon. The polysilicon thin film transistors are advantageous in that they have both data and gate driver circuits manufactured with a pixel array on a glass substrate and, when applied to a pixel device, they can provide high image quality and aperture rate because switching devices can be miniaturized.
One of the most important techniques for fabricating low temperature polysilicon thin film transistors is crystallization using an eximer laser applied to an amorphous silicon thin film at a low temperature of below 400.degree. C.
This technique melts amorphous silicon with a radiated energy, such as laser, within a short time and cools the molten matter to extract crystals. Small crystal seeds initially produced are grown to crystalline aggregates. Growth conditions determine the growth orientation of crystals, which leads to single-crystallization in a single growth direction, or poly-crystallization in multiple directions. The energy uniformity of a laser beam applied to the amorphous silicon is also a factor that determines crystallization properties, such as crystal size, roughness of crystal surface and crystal orientation.
FIG. 1 is a schematic illustration of a conventional laser annealing apparatus. A conventional laser annealing apparatus is divided into two parts: a laser generating and setting device I for generating a laser beam and measuring the energy value of the laser beam, which energy value is converted to an electrical signal and compared to a predetermined reference value, and applying a voltage as high as the difference between the measured and reference values, so as for the laser to be set to a desired energy value; and a process device II for applying the laser beam of the desired energy value set by the laser generating and setting device I to a sample, such as amorphous silicon.
In the conventional laser annealing apparatus, the laser generating and setting apparatus I includes a laser generating section (a), an energy converting section (b) and an energy control section (c) In particular, the laser generating section (a) produces a laser beam from injected sources which are decomposed by the applied voltage and undergo a reaction. The energy converting section (b) has an energy probe for detecting heat energy of the laser beam emitted from the laser generating section (a), and a laser energy meter for converting the detected heat energy to an electrical signal. The energy control section (c) includes a comparator for comparing the electrical signal output from the laser energy meter with a reference value, and a power supplier for applying a voltage to the laser generating section (a) based on the output signal of the comparator.
The process device II, which is connected to the laser generating section (a) of the laser generating and setting device II and applies a laser beam to the amorphous silicon, comprises a homogenizer for making the set energy density of the laser beam uniform, and a process window through which the laser beam homogenized is applied to a sample 120, amorphous silicon.
A process for applying a laser beam of a desired energy value to the amorphous silicon with the conventional laser annealing apparatus of the aforementioned construction will be described as follows.
When injecting Xe and HCl gases into the laser generating section (a) and applying a required voltage through the power supply, the gases undergo the following reaction I to produce a laser beam (XeCl) having a short wavelength. EQU Xe+HCl.fwdarw.XeCl (I)
The laser beam passes through a first window 102 formed at the one end of the laser generating section (a), being amplified and reflected through a first lens 106 which is arranged in parallel with the first window 102. After the first lens 106, the laser beam passes through a second lens 108 via a window 104 which is located at the other end of the laser generating section (a). A splitter 110 is adapted for 99% of the laser beam to be passed through and for the rest of the beam to be reflected.
About 1% of the laser beam is transmitted to the energy converting section (b) by the splitter 110, while the rest 99% is transmitted to the process device II.
At this stage, the percentage of laser beam reflection or transmittance depends on the composition of the layer coated on the surface of the splitter 110.
The energy probe senses heat of the 1% portion of the laser beam reflected from the splitter 110 and calculates the heat energy interms of energy units, which is output as an electrical signal through to laser energy meter.
This output signal is applied to the comparator and compared with a reference value. The electrical signal output from the comparator is digitized and sent to the power supplier. The power supplier applies a voltage corresponding to the digitized electrical signal to the laser generating section (a).
The laser generating section (a) produces a laser beam with an energy value depending on the applied voltage. Once the laser beam is set to a desired energy value in the manner as described above, the remaining 99% portion of the laser beam is transmitted to the process device II through the slit 110.
A process for applying this 99% portion of the laser beam at the process device II to amorphous silicon will be described as follows. After passing through the splitter 110, the direction of the laser beam is changed by an angle through mirrors 114, 116 and 118 and transmitted to the homogenizer.
Generally, a laser beam has a high energy density in the center relative to the peripheral part and has a Gaussian-shaped energy distribution as shown in FIG. 2. The homogenizer sets the energy distribution of the laser beam such that the top energy distribution is substantially flat and even, as illustrated in FIG. 3. As the laser beam having such an even energy distribution is irradiated on the sample 120 through the window, the amorphous silicon undergoes crystallization.
However in the prior art, as the laser process is repeated over and over, the internal parts of the apparatus, such as a plurality of mirrors, a slit, lenses of a homogenizer, and a process window are susceptible to contamination and damages. Thus a use of damaged apparatus may cause a laser generating and setting device to apply a laser beam of inaccurate energy value to the amorphous silicon.